1. Field of the Invention
The present invention relates to a hybrid vehicle and, more particularly, to a hybrid vehicle provided with a generator-motor capable of imparting torque to front wheels of the vehicle and another generator-motor capable of imparting torque to rear wheels thereof.
2. Description of the Related Art
A hybrid vehicle (so-called parallel hybrid vehicle) having an engine and a generator-motor, which is a motor capable of acting as a generator and also as a motor, as a driving source of the vehicle, normally performs regenerative operation of the generator-motor when the vehicle is slowing down so as to operate the generator-motor as a generator and to charge a battery (secondary battery) with the generated energy, the battery serving as a power source of the generator-motor. Thus, a part of the kinetic energy of the vehicle is converted into electric energy to be collected or absorbed by the battery. As a technique for performing the regenerative operation of a generator-motor when a vehicle slows down, there has been known a technique disclosed in, for example, the Japanese Unexamined Patent Application Publication No. 2000-316204 (hereinafter referred to as “patent document 1”).
The patent document 1 has discloses a hybrid vehicle in which a rotating shaft (rotor) of a generator-motor is directly coupled to a crankshaft of an engine, and the generated torque of the engine and the torque of the generator-motor are output to wheels through the intermediary of a transmission. According to the technique described in the patent document 1, the regenerative operation of the generator-motor is performed when a brake pedal of the vehicle is operated. At this time, as the operating amount of the brake pedal increases, the regenerative current of the generator-motor increases. Thus, when the vehicle decelerates, the ratio of the kinetic energy of the vehicle that is lost due to mechanical frictional heat is decreased, while the ratio of the kinetic energy to be converted into electric energy is increased so as to improve the effective use of the energy.
There has been known another type of hybrid vehicle. This type of hybrid vehicle is provided with an additional generator-motor (hereinafter referred to as “the second generator-motor”) capable of independently imparting torque to different driving wheels (e.g., rear wheels) from the driving wheels (e.g., front wheels) to which output torque of the engine and the generator-motor is imparted, in addition to the generator-motor (hereinafter referred to as “the first generator-motor”) connected to the engine and the transmission, as in the case of the vehicle described in the patent document 1. In such a hybrid vehicle, both the first and second generator-motors are capable of performing regenerative operation when the vehicle slows down, so that how to successfully have the two generator-motors share the braking force of entire vehicle based on the regenerative operations becomes an important consideration.
This is typically handled as described below. When, for example, the vehicle is decelerated by braking, of all braking force of the vehicle, the braking force to be provided by the front wheels of the vehicle and the braking force to be provided by the rear wheels are determined according to a predetermined distribution ratio based on longitudinal weight distribution in the vehicle. Then, based on the determined braking forces, a friction type braking mechanism of the front wheels and a friction type braking mechanism of the rear wheels are controlled. Hence, the hybrid vehicle having the first and second generator-motors described above could be adapted so that the ratio of the braking force to be produced by the regenerative operation of the first generator-motor and the ratio of the braking force to be produced by the regenerative operation of the second generator-motor out of a total braking force required by the whole vehicle when the vehicle slows down are determined in advance on the basis of the longitudinal weight distribution of the vehicle.
However, the aforementioned method for determining the braking force distribution ratios between the generator-motors poses the following difficulties in achieving improved efficiency in use of energy.
Regarding the weight distribution of a vehicle, of the front wheels and the rear wheels, the pair of wheels equipped with an engine and the first generator-motor (hereinafter the front wheels in this case) are generally heavier than the other pair of wheels (hereinafter the rear wheels in this case). If, therefore, the braking force distribution ratio between the generator-motors is determined on the basis of the weight distribution of the vehicle, then the first generator-motor on the front wheel side will basically assume a larger share than the second generator-motor on the rear wheel side. However, the first generator-motor is coupled to the vehicular front wheels through the intermediary of a transmission, so that the efficiency of torque transmission between the first generator-motor and the front wheels is generally lower than that between the second generator-motor and the rear wheels. In addition, the engine is connected to the first generator-motor, so that a part of the kinetic energy of the vehicle transmitted from the front wheels of the vehicle to the first generator-motor is undesirably absorbed as a frictional heat inside the engine. This tends to lead to deteriorated efficiency of conversion of the vehicular kinetic energy into electric energy, making it difficult to improve the use efficiency of energy.